<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <!-- 通过对话框输入圆的半径，计算出圆的面积 -->
    <script>
        let radius = prompt('请输入圆的半径')
        const PI = 3.14
        let area = PI * radius*radius
        document.write('圆的面积为：'+area)
    </script>
</body>
</html>